Polymers containing alkali metal termination groups can be treated with various functional reagents, including both difunctional and polyfunctional, to result in coupled linear or branched polymers of increased molecular weight. These coupling processes provide polymers exhibiting improved processing or other properties attributable to the increase in molecular weight and also to the branching.
A variety of coupling agents have been employed, which contain functional groups including ester groups. It is known that a monoester would be expected to produce linear coupling, or that a coupling agent including two ester groups derived from a dicarboxylic acid would be expected to produce a coupled polymer containing four branches, and so on.